The present invention provides a new and improved lead-free and cadmium-free glass enamel composition for use in decorating glass articles such as, for example, architectural glass plate.
Various lead-free and cadmium-free glass enamel compositions are known in the prior art. Examples include the compositions described in Ruderer, et al., U.S. Pat. No. 5,306,674; Heitmann, et al., U.S. Pat. No. 5,707,909; Clifford, U.S. Pat. No. 5,304,516; Klimas, et al., U.S. Pat. No. 4,970,178; Anquetil, et al., U.S. Pat. No. 5,350,718; and Anquetil, U.S. Pat. No. 5,674,789. Such enamel compositions are often applied in the form of a paste to a glass surface to be enameled. The paste usually contains finely divided particles of one or more glassy materials, commonly called xe2x80x9cglass frits,xe2x80x9d one or more colorants or pigments, fillers, and a vehicle. After application of the paste by screen printing or other techniques, the composition is fired to volatilize and/or thermally decompose and drive off the vehicle, fuse the glass frits, pigments, and fillers together, and bond the enamel to the glass surface.
For many applications, a lead-free and cadmium-free glass enamel composition must display good flow at traditional glass enamel firing temperatures (typically from about 550xc2x0 C. to about 720xc2x0 C.). Upon firing, such enamel compositions preferably produce an enamel finish that displays no cracking, good chemical resistance, and good weatherability. Several prior art glass enamel compositions can be used to produce enamels exhibiting these properties, but there is a need for a lead-free and cadmium-free glass enamel composition that exhibits improved color values upon firing in addition to these properties.
The present invention provides a new and improved lead-free and cadmium-free glass enamel composition that displays good flow at traditional glass enamel firing temperatures and upon firing produces an enamel finish that displays no cracking, good chemical resistance, good weatherability, and improved color values. The glass enamel composition of the present invention comprises a glass component, an oxidizer, a pigment system, and optional fillers. The glass component preferably comprises one or more glass frits comprising from about 28% to about 60% by weight SiO2, from about 9% to about 21% by weight B2O3, up to about 15% by weight Na2O , up to about 13% by weight K2O , up to about 6% by weight Li2O, up to about 6% by weight Al2O3, up to about 25% by weight TiO2, from about 5% to about 22% by weight ZnO, up to about 20% by weight Nb2O5, up to about 9% by weight Bi2O3, up to about 11% by weight ZrO2, up to about 12% by weight BaO, up to about 8% by weight SrO, up to about 5% by weight CaO, up to about 2% by weight MgO, up to about 5% weight LnOx, where xe2x80x9cLnxe2x80x9d is an element selected from the Lanthanide Series and is most preferably an element selected from the group consisting of La, Ce, Pr, and Nd, up to about 6% by weight Sb2O3, and up to about 6% by weight fluorine. The presence of the oxidizer in the composition, which can be included as part of the glass component, as a separately added material, or both, significantly improves the color values of the fired enamel as compared to enamels formed using similar glass enamel compositions having no oxidizer present. When the oxidizer is included as part of the glass component, the oxidizer preferably comprises Sb2O3, NH4NO3, or K2S2O8. Alternatively, the oxidizer can be included as part of the glass component as one or more low temperature decomposing nitrates such as NaNO3 and KNO3, persulfates such as (NH4)2S2O8 and K2SO4, peroxides such as BaO2 and ZnO2, antimonates such as NaSbO3, CaSb2O6 and sulfates such as Na2SO4, and for colored glasses, as one or more chromates and/or permanganates. When the oxidizer is included as a separately added material, the oxidizer preferably comprises one or more compounds selected from the group consisting of peroxides such as barium peroxide, zinc peroxide, and hydrogen peroxide, and persulfates such as ammonium persulfate, and potassium persulfate. A broad range of pigment systems can be used in the invention, including white pigments systems based upon TiO2, CeO2, ZrO2, and zircon, and colored pigment systems conventionally used to produce colors such as blue, brown, and black.
The foregoing and other features of the invention are hereinafter more fully described and particularly pointed out in the claims, the following description setting forth in detail certain illustrative embodiments of the invention, these being indicative, however, of but a few of the various ways in which the principles of the present invention may be employed.
The present invention provides a new and useful lead-free and cadmium-free glass enamel composition that displays good flow at traditional glass enamel firing temperatures. Upon firing, the composition of the present invention forms an enamel finish having good chemical resistance, no cracking, good weatherability, and improved color values. As noted above, a need exists in the glass enamel industry for a lead-free and cadmium-free glass enamel composition that exhibits these properties.
The glass enamel composition of the present invention is lead-free and cadmium-free. As used throughout this specification in the claims below, the terms xe2x80x9clead-freexe2x80x9d and xe2x80x9ccadmium-freexe2x80x9d mean that no lead, or PbO, cadmium, or CdO, have been intentionally added to the composition, and that the composition comprises less than about 0.5% by weight PbO or CdO.
The glass enamel composition of the present invention comprises a glass component, an oxidizer, a pigment system, and optional fillers. The glass enamel composition according to the present invention is preferably mixed with a suitable vehicle or carrier which allows the enamel composition to be applied to a section of glass in a convenient form such as, for example, a slurry, a paste, or a thermoplastic pellet.
The glass component of the glass enamel composition of the present invention preferably comprises one or more glass frits. Preferably, the glass component of the present invention displays the following composition:
The glass frits employed in the present invention may be prepared utilizing conventional glass melting techniques, which are well known in the art. Typically, a 500 g. batch is smelted at around 1250xc2x0 C.xc2x1100xc2x0 C. for about 30-60 minutes and the molten glass is then converted to frit by water quenching or using water-cooled rollers. It will be appreciated that production of the glass frits used in the invention is conventional, and that any of the various techniques well known to those skilled in the art can be employed.
As noted, the glass enamel composition according to the present invention also comprises an oxidizer. The oxidizer contributes to the improved color values of the enamel upon firing. The oxidizer may be included in the glass component, or it may be included in the enamel composition as a separately added material. Alternatively, the oxidizer may be included both in the glass component and as a separately added material. In all cases, the oxidizer should be stable at room temperatures in the presence of the other components that comprise the composition.
When the oxidizer is included in the glass component, it preferably comprises one or more compounds selected from the group consisting of Sb2O3, NH4NO3, and K2S2O8. Alternatively, the oxidizer can be included as part of the glass component as one or more low temperature decomposing nitrates such as NaNO3 and KNO3, persulfates such as (NH4)2S2O8 and K2SO4, peroxides such as BaO2 and ZnO2, antimonates such as NaSbO3 and CaSb2O6 and sulfates such as Na2SO4, and for colored glasses, as one or more chromates and/or permanganates.
When the oxidizer comprises Sb2O3 included in the glass component, it is preferable that the glass component be processed in an oxidizing atmosphere to assist in converting the Sb2O3 to Sb2O4, which serves as the oxidizer during subsequent firing. Production scale furnaces typically include means for controlling the composition of the processing atmosphere. However, it will be appreciated that nitrates can be included in the Sb2O3 containing batch materials as an alternative to processing the glass component in an oxidizing atmosphere. This is a particularly useful alternative when preparing small laboratory scale batches using an electric furnace in air.
The amount of oxidizer to be smelted into the glass will depend, in part, upon whether an oxidizer is also being included in the enamel composition as a separately added material. Although greater amounts of oxidizer can be used without adverse consequences, generally an amount comprising from about 0.25% to about 6% by weight of the glass component is sufficient to obtain the desired improved color values.
When the oxidizer is included in the composition as a separately added material, preferably it comprises one or more compounds selected from the group consisting of peroxides such as barium peroxide, zinc peroxide, and hydrogen peroxide, and persulfates such as ammonium persulfate, and potassium persulfate. It will be appreciated that the composition of the oxidizer is not per se critical, and any compound or combination of compounds that provide oxidation during firing and which do not interfere with enamel formation can be used in the invention. The oxidizer can be separately added to the enamel composition during milling as a mill addition, as part of the vehicle or carrier, or to the paste subsequent to milling.
Although in the preferred embodiment of the invention the oxidizer comprises a material which is specifically added to the glass component or separately added to the enamel composition to provide oxidation during firing, the desired improved color values can be also be obtained within the scope of the invention by including an oxidizer in the glass component as a remnant. One method of creating an oxidizer in the glass component as a remnant is by bubbling oxygen through the molten glass during processing, a portion of which will be captured and retained in the solid glass when the molten glass is converted to frit.
It is believed that the presence of the oxidizer in the enamel composition according to the present invention improves the color values of the fired enamel by keeping transition elements such as, for example, titanium, niobium, iron and zinc, in their most oxidized state. By keeping these transition elements in their most oxidized state, the oxidizer prevents the formation of undesired color centers in the fired enamel. It is also believed that the presence of the oxidizer in the composition according to the present invention prevents undesired glass/pigment interactions from occurring, which can also lead to the development of undesired color formation.
In addition to the glass component and oxidizer, the glass enamel composition according to the present invention also includes a pigment system. For white enamels, the preferred white pigment system comprises TiO2. While other white pigment systems can successfully be used in the invention, such as for example, CeO2, ZrO2, and zircon, TiO2 is nevertheless preferred because it has a higher refractive index (both anatase and rutile) than most other white pigment systems, and TiO2 is readily available. Colored pigment systems can also be used in the invention. Examples of potential suitable color pigments include copper chrome black sold under the trade designation K-393-2, iron nickel manganese chrome black sold under the trade designation V792, cobalt aluminate blue sold under the trade designation V-3285, zinc iron chrome brown sold under the trade designation K-2102, and iron cobalt chrome black sold underthe trade designation F-6340, all sold by the Ferro Corporation of Cleveland, Ohio.
The glass enamel composition according to the present invention also optionally includes one or more fillers. Examples of suitable fillers include alumina, zircon, cordierite, willemite, beta-eucryptite, spodumene, and silicon dioxide. It will be appreciated that the composition of the optional filler is not per se critical, and that any desired filler that does not adversely affect the enamel firing temperature can be used in the invention.
The glass enamel composition according to the present invention is utilized in the production of a section of formed decorated glass by first mixing the glass component, oxidizer, pigment system, and one or more optional fillers and vehicles, if so desired, so as to produce a form appropriate for application of the enamel composition to a section of glass. The solids portion of the enamel composition comprises at least about 40% by weight of the enamel composition. As used in this specification and in the claims below, the term xe2x80x9csolids portionxe2x80x9d means that part of the glass enamel composition that survives firing. Preferably, the solids portion comprises at least about 60% by weight of the enamel composition. More preferably, the solids portion comprises at least about 70% by weight of the enamel composition.
Preferably, the glass component comprises at least about 15% by weight of the solids portion of the glass enamel composition. More preferably, the glass component comprises from about 25% by weight to about 95% by weight of the solids portion of the glass enamel composition. When the oxidizer is a separately added material, the oxidizer enamel composition. The pigment system generally comprises from about 5% to about 30% of the solids portion of the glass enamel composition according to the present invention. Any optional fillers used in the composition generally comprise up to about 30% by weight of the solids portion of the enamel composition.